The present invention relates to an electrostatic recording method and apparatus used in machines such as printers and facsimile machines. More particularly, it is directed to an electrostatic recording method and apparatus for electrostatically recording images by emitting ions in accordance with an image signal.
Conventional electrostatic recording apparatuses of this type include the following. As shown in FIG. 1, an electrostatic recording head 101 is disposed so as to confront a dielectric drum 100 and it forms a desired latent electrostatic image on the dielectric drum 100.
As shown in FIGS. 2 and 3, the electrostatic recording head 101 has a plurality of drive electrodes 103, 103, . . . in parallel with each other on the front surface of an insulating substrate 102 and a plurality of control electrodes 104, 104 . . . so as to intersect the drive electrodes 103, 103, . . . on the back surface thereof so that a matrix is formed by both electrodes 103, 103, . . . and 104, 104, . . . . On the control electrodes 104, 104, . . . are opening portions 105, 105, . . . serving as discharge generating regions at such positions as to intersect the drive electrodes 103, 103, . . . formed, respectively. Since these opening portions 105, 105, . . . formed on the control electrodes 104, 104, . . . are arranged at positions where the drive electrodes 103, 103, . . . and the control electrodes 104, 104, . . . , both constituting the matrix, intersect each other, the opening portions 105, 105, . . . themselves become part of the matrix as shown in FIG. 2.
As shown in FIGS. 4 and 5, on the lower surface of the control electrodes 104, 104, . . . is a screen electrode 107 formed through an insulating layer 106, while the insulating layer 106 and the screen electrode 107 have, as shown in FIG. 5, opening portions 108, 108, . . . and ion guiding opening portions 109, 109, . . . at positions corresponding to the opening portions 105, 105, . . . of the control electrodes 104, 104, . . . . As shown in FIG. 5, the electrostatic recording head 101 applies not only an ac voltage to the drive electrode 103 but also a dc voltage to the screen electrode 107. The head 101 also applies a pulsed high voltage selectively to the control electrodes 104, 104, . . . in accordance with an image signal.
Accordingly, as shown in FIG. 6, creeping corona discharge occurs at the opening portions 105, 105, . . . between the drive electrodes 103, 103, . . . and the control electrodes 104, 104, . . . to which the high voltage has been selectively applied. And a stream of ions generated by the creeping corona discharge is either accelerated or absorbed by an electric field generated between the control electrodes 104, 104, . . . and the screen electrode 107, thereby causing a latent electrostatic image to be formed on the dielectric drum 100 by the ions in accordance with the image signal.
To form a desired latent electrostatic image on the dielectric drum 100, emission of the ions must be controlled. Since the ions are emitted from the opening portions 105, 105, . . . and these opening portions 105, 105, . . . constitute the matrix, such control naturally involves the drive electrodes 103, 103, . . . and the control electrodes 104, 104, . . . , which will be driven in the following manner in synchronism with rotation of the dielectric drum 100.
As shown in FIG. 1, an encoder 110 is fixed on the rotating shaft of the dielectric drum 100 and it detects the speed of rotation or position of the dielectric drum 100. As shown in FIG. 7, while one period T of a pulse is being outputted from the encoder 110, the plurality of drive electrodes 103, 103, . . . are sequentially driven by a predetermined pulse width Tw every electrode, and a pulsed voltage is applied to the control electrodes 104, 104, . . . for recording the latent electrostatic image in synchronism therewith.
In this way, the creeping corona discharge occurs at the opening portions 105, 105, . . . between the drive electrodes 103, 103, . . . and the control electrodes 104, 104, . . . . to which the voltage has been applied; the stream of ions produced by the creeping corona discharge is accelerated or absorbed by the electric field generated between the control electrodes 104, 104, . . . and the screen electrode 107; the emission of the ions are controlled; and the latent electrostatic image is formed on the dielectric drum 100 in accordance with the image signal.
For example, to record a linear image by the electrostatic recording head 101, not only the ac voltage is applied to the drive electrodes 103, 103, . . . sequentially in synchronism with the rise of a pulse outputted from the encoder 110, but also the pulsed voltage is applied to the control electrodes 104, 104, . . . in synchronism with driving each of the drive electrodes 103, 103, . . . as shown in FIG. 7. Specifically, a control electrode is turned on when a No. 1 drive electrode has been driven. Then, when the latent electrostatic image formed at that timing has reached a No. 2 drive electrode as the dielectric drum 100 moves, the control electrode is turned on (while the dielectric drum 100 is moving, the control electrode is kept off). By repeating this operation, the linear latent electrostatic image is formed on the dielectric drum 100 by movement of the dielectric drum 100 as shown in FIG. 8.
The latent electrostatic image thus formed on the dielectric drum 100 is developed by a developing unit 111 shown in FIG. 1 to form a toner image, and this toner image is transferred and simultaneously fused by pressure onto a recording sheet 113 supplied to a nip portion between the dielectric drum 100 and a pressure roller 112 that is in pressure contact therewith. As a result, the image is recorded on the recording sheet 113. In FIG. 1, reference numeral 114 designates a cleaner that removes toner remaining on the surface of the dielectric drum 100 after the toner image has been transferred and fused as described above; and 115, a discharge unit for removing charge remaining on the surface of the dielectric drum 100.
However, the above prior art imposes the following problems. The electrostatic recording apparatus transfers and fuses the toner image formed on the dielectric drum 100 surface onto the recording sheet 113 by pressure from both the dielectric drum 100 and the pressure roller 112 that is in pressure contact therewith. As a result, when the recording sheet 113 is threading into the nip portion between the dielectric drum 100 and the pressure roller 112 as shown in FIG. 9(a), the contact pressure between the dielectric drum 100 and the pressure roller 112 is increased instantaneously, causing the speed of rotation of the dielectric drum 100 to be decreased instantaneously as shown by A in FIG. 10 and then returned to the original speed after some vibration. Thereafter, when the recording sheet 113 passes through the nip portion between the dielectric drum 100 and the pressure roller 112 as shown in FIG. 9(b), the speed of rotation of the dielectric drum 100 is maintained at a predetermined steady-state level as shown by B in FIG. 10. Similarly, when the recording sheet 113 exits from the nip portion between the dielectric drum 100 and the pressure roller 112 as shown in FIG. 9(c), the contact pressure between the dielectric drum 100 and the pressure roller 112 is decreased instantaneously, causing the speed of rotation of the dielectric drum 100 to be increased instantaneously as shown by C in FIG. 10 and then returned to the original speed after some vibration.
Thus, the speed of rotation of the dielectric drum 100 undergoes a drastic change as the recording sheet 113 passes through the nip portion between the dielectric drum 100 and the pressure roller 112. As a result, with respect to the pulse outputted from the encoder 110 which determines the timing for driving the electrostatic recording head 101 by detecting the speed of rotation of the dielectric drum 100, its period T also undergoes a change as the recording sheet 113 passes through the nip portion as shown in FIG. 11.
In contrast thereto, the output of an electrostatic recording head 101 drive signal is started in synchronism with the rise of the pulse outputted from the encoder 110 as shown in FIG. 7, and is then continued in the form of a pulse signal so that the drive electrodes 103, 103, . . . can be driven sequentially at the predetermined pulse width Tw and interval Td.
As a result, when the speed of rotation of the dielectric drum 100 becomes lower than a predetermined speed as the recording sheet 113 passes through the nip portion and the period of the pulse outputted from the encoder 110 becomes longer than a predetermined interval, the head drive signal becomes relatively shorter by .DELTA.t1 as shown in FIG. 11. Similarly, when the speed of rotation of the dielectric drum 100 becomes higher than the predetermined speed and the period of the pulse outputted from the encoder 110 becomes shorter than the predetermined interval as shown in FIG. 11, the head drive signal becomes relatively longer by .DELTA.t2.
As a result, the interval of the dot-like ions emitted from the opening portions 109, 109, . . . on the electrostatic recording head 101 driven by the head drive signal desynchronizes with rotation of the dielectric drum 100, causing irregular distortion to the image to be recorded on the dielectric drum 100 as the recording sheet 113 passes through the nip portion as shown in FIG. 12 (in case of recording, e.g., a Chinese character " " by the electrostatic recording head 101), with resultant impairment in image quality.
Further, the above prior art apparatus entails the following problems. The head drive signals received by the drive electrodes 103, 103, . . . and the control electrodes 104, 104, . . . include drive electrode signals and control electrode signals as shown in FIG. 7, and the ions are emitted in the form of dots only from the opening portions 109, 109, . . . to which both electrode signals have been applied simultaneously to form a latent electrostatic image.
In FIG. 13, when a first drive electrode 103 and a first control electrode 104 receive a pulsed voltage simultaneously, a dot d1 is printed. And as shown in FIG. 7, during one period T of a pulse outputted from the encoder 110, the drive electrode signal 111 drives the first to fifth drive electrodes 103, 103, . . . sequentially at a predetermined pulse width Tw and a predetermined pulse interval Td, driving the control electrodes 104, 104, . . . in synchronism with the drive electrode signal 111, while sequentially printing dots d1, d2, d3, d4, and d5. Adjacent to the dots d1, d2, d3, d4, d5 are dots d1', d2', d3', d4', d5' printed simultaneously therewith.
When a next pulse is outputted from the encoder 110 as the dielectric drum 100 rotates as shown in FIG. 7, the drive electrodes 103 are driven sequentially starting with the first drive electrode in a manner similar to the above. Since the dielectric drum 100 is being rotated, a dot d6 is printed at a position in a dielectric drum 100 rotating direction A adjacent to the already recorded dot d1 by the electrostatic recording head 101 as shown in FIG. 13. Thereafter, as the dielectric drum 1 further rotates, dots such as dots d11, d16, d21, d26, d31, d36, d41 are sequentially printed in the matrix form in the dielectric drum 100 rotating direction A every time a pulse is outputted from the encoder 110 on a period basis, thereby recording the desired image. As a result, the forty-first dot d41 comes into line contiguous to the fifth dot d5' printed by the first pulse outputted from the encoder 110.
By the way, upon output of a pulse from the encoder 110, the positions of the dots d1, d2, . . . d5 are basically defined by the pitch of the drive electrodes 103, 103, . . . on the electrostatic recording head 101, while the positions of the dots d1, d6, d11, . . . , which are sequentially recorded in the dielectric drum 100 rotating direction A every time a pulse is outputted from the encoder 110, are defined by the pitch of the pulses outputted from the encoder 110.
Any variation in the pitch of the drive electrodes 103, 103, . . . due to inaccuracy in their fabrication, error in the number of pulses outputted from the encoder 110, or any dimensional error in the diameter of the dielectric drum 100 may cause incoincidence between the pitch d of the dots printed in accordance with the adjacent drive electrodes 103, 103, . . . on the electrostatic recording head 101 and the value obtained by multiplying by a predetermined integer n (n=2 in an example shown in FIG. 21) the pitch P of the dots sequentially printed in accordance with the pulses outputted from the encoder 110 as shown in FIG. 13. Therefore, for example, the dot d41 that should be printed contiguous to the dot d5' is printed out of place from the dielectric drum 100 rotating direction A. As a result, in recording the Chinese character " " such as shown in FIG. 14, the horizontal line of the character appears as being regularly saw-toothed, thereby greatly impairing the image quality.
This problem can be analyzed quantitatively as follows.
Let it be supposed that one period of a pulse outputted from the encoder 110 is T and that the time required for recording a single dot by the electrostatic recording head 101 is TD (=Tw+Td). Then, T and TD can be expressed as follows. EQU T=P/v (1) EQU TD=T/N=P/Nv (2)
where P is the pitch of the dots printed contiguously in the dielectric drum 100 rotating direction A, v is the circumferential velocity of the dielectric drum 100, and N is the number of drive electrodes 103, 103, . . . as shown in FIG. 13.
If the drive electrodes 103, 103, . . . are sequentially driven at the predetermined pulse width Tw and pulse interval Td to record the dots d1, d2, d3, . . . d5 in the order as written, the interval d between the dots d1 and d2 to be recorded on the dielectric drum 100 will be set to a multiple of an integer n (n=2 in FIG. 13) of the pitch P between the dots, because the dielectric drum 100 rotates within that time. However, this pitch d is equal to a value obtained by adding a distance for which the dielectric drum 100 moves during the time TD to a geometrical interval d' between the drive electrodes 103, 103, . . . on the electrostatic recording head 101. Thus, the interval d can be expressed as follows in consideration of equation (2). EQU d=nP =d'+vTD=d'+P/N (3).
Hence, the pitch P can be defined as follows by modifying equation (3). EQU P=d'/(n-1/N) (4).
On the other hand, if the diameter of the dielectric drum 100 is Q, the number of pulses generated per one full rotation of the encoder 110 is NE, the pitch P' determined by the pulse outputted from the encoder 110 is given as follows. EQU P'=.rho.l/NE (5).
Here, if the pitch P between the dots to be printed contiguous to each other in the dielectric drum 100 rotating direction A coincides with the pitch P' determined by the pulse outputted from the encoder 110, there will be no dots which are out of place in the printed image.
Let us think about the case where a 10-dots/mm printing is performed on a dielectric drum 100 whose diameter is 200 mm using an electrostatic recording head 101 in which the interval d' between the drive electrodes 103, 103, . . . viewed from the dielectric drum 100 is 0.2 mm, n is 2, and the number N of drive electrodes 103, 103, . . . is 5. If the number of pulses NE outputted per rotation of the encoder 110 is 6000, then the pitch P to be given by equation (4) is as follows. EQU P=0.2/(2-1/5).apprxeq.0.111.
The pitch P' to be given by equation (5) is as follows. EQU P'=200.rho./6000.apprxeq.0.105.
Hence, there results a difference of 0.006 mm between both pitches P and P', causing a dislocation which is 8 times that difference, i.e., 0.048 mm, between the dot 41 and the dot d5'.
This dislocation is brought about by incoincidence between the pitch P of the dots printed contiguous to each other in the dielectric drum 100 rotating direction A and the pitch P' determined by the pulse outputted from the encoder 110.
Therefore, as the dislocation is aggravated with increasing variation in the interval d' between the drive electrodes 103, 103, . . . on the electrostatic recording head 101 and with increasing errors of the number NE of pulses outputted from the encoder 110 or of the diameter of the dielectric drum 100.
As a result, the horizontal line of the character is printed regularly saw-toothed when an image is recorded, thereby imposing the problem of greatly impairing the image quality.